In conventional optical waveguide sensor assemblies, it is difficult to determine the precise nature of any change in the effective refractive index of the sensor component i.e. the factors such as changes in dimension (e.g. physical thickness) and/or changes in composition (i.e. intrinsic refractive index) which contribute to changes in effective refractive index cannot be differentiated. This may be problematic for a number of reasons. For example, as a consequence of vapour ingress into an absorption layer, the contribution of changes in intrinsic refractive index may be opposite to the contribution of changes in refractive index due to physical swelling. This not only reduces the effectiveness of the sensor component but also ensures that the aggregate response is a non-linear function of the vapour concentration.
Generally speaking, it is known to make use of the evanescent field component of electromagnetic radiation incident on a waveguide structure (i.e. the field which extends outside the guiding region) to sense discrete changes in optical properties (see inter alia GB-A-2228082, U.S. Pat. No. 5,262,842, WO-A-97/12225 and GB-A-2307741). This technique relies on “leakage” of optical signals from a waveguide into a sensing layer typically formed from an absorbent polymer. The evanescent component of the optical signal being guided by the waveguide is typically small leading to limited interrogation of the sensing layer. Surface plasmon resonance techniques are also well known to those skilled in the art. Both of these techniques are frequently used in a “difference” mode i.e. where TE (transverse electric) and TM (transverse magnetic) modes are excited simultaneously to determine changes in effective refractive index. In the case of surface plasmon resonance techniques, it is the interaction solely of the TM mode with the analyte that is detected. In neither case has the interaction of both the TE and TM modes been exploited to differentiate factors contributing to changes in effective refractive index.
Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings.